The present invention generally relates to a pickup device for optically reading signals recorded on a record medium such as a so-called compact disc or the like, and more particularly, to an optical pickup device arranged to divide or split a light path and also to produce astigmatism through employment of a half-mirror.
Conventionally, there has been provided a device arranged to optically read signals recorded on a record medium, for example, in the form of a record disc through employment of an optical pickup. As one typical example of the optical pickup devices as referred to above, there is known a so-called CD player adapted to reproduce or play a compact disc (i.e. CD) having, on its signal surface, dents or concave portions called pits continuously formed to correspond to digital signals. In the CD player as referred to above, it is necessary to effect a focusing control in which a light beam produced by the optical pickup device is controlled in a direction of an optical axis or a so-called focusing direction in order to accurately converge the light beam onto the signal surface of the disc, and also, to effect a tracking control in which the light beam is controlled in a radial direction of the disc or a so-called tracking direction in order to cause the light beam to follow the signal track on the signal surface of the disc.
Incidentally, in the CD player, a practice to control positions of an objective lens for the optical pickup device has been generally adopted for the focusing control and tracking control, and it has been a recent trend that many CD players employ an astigmatic method for the focusing control, and a three-beam method for the tracking control as disclosed, for example, in Japanese Patent Laid-Open Application Tokkaisho No. 60-209974.
In the astigmatic method as referred to above, it is so arranged that, through employment of a cylindrical less and a half-mirror, astigmatism is produced in the light reflected by the signal surface of the disc and passing through a predetermined light path within the optical pickup device, and such reflected light is irradiated onto a main detecting portion D1 of a photo-detector having four sensing portions or sensors symmetrically divided by two axes or dividing lines D1a as shown in FIG. 8, thereby to detect focusing deviation onto the signal surface by the light beam through calculation processing of outputs of the respective sensors of said main detecting portion D1.
More specifically, in the case where the focusing point of the light beam is aligned with the signal surface of the disc, i.e. in the state of in-focus, the light spot of the reflected light irradiated onto the main detecting portion D1 of the photo-detector is in a state as shown in a solid line in FIG. 8, with outputs S1, S2 S3 and S4 of the respective sensors becoming equal to each other. Meanwhile, when the focusing point of the light beam is located in a position before the signal surface of the disc, the light spot of the reflected light as projected onto the main detecting portion D1 of the photo-detector is in a state as shown in a one-dot chain line (or two-dot chain line) in FIG. 8, and the outputs S1 and S3 (or S2 and S4) of the sensors disposed on a diagonal line are large, with the outputs S2 and S4 (or S1 and S3) becoming small. On the contrary, in the case where the focusing point of the light beam is located at a position behind the signal surface of the disc, the light spot of the reflected light as projected onto the main detecting portion D1 of the photo-detector is in a state as shown in the two-dot chain line (or one-dot chain line) in FIG. 8, and the outputs S2 and S4 (or S1 and S3) of the sensors disposed on the diagonal line are large, with the outputs S1 and S3 (or S2 and S4) becoming small. Therefore, by effecting calculation processing for (S1+S3) - (S2+S4) of the respective sensor outputs S1, S2, S3 and S4 of the main detecting portion D1, it is possible to obtain a focusing error signal indicative of deviation in the focusing by the light beam onto the signal suface, and thus, the focusing deviation on the signal surface by the light beam can be detected.
Meanwhile, in the three-beam method, it is so arranged that a light beam (laser light) emitted from a light source (generally of a laser diode) is diffracted through employment of a diffraction grating so as to prepare two sub-beams from said light beam besides the main beam, and the light of the sub-beams reflected by the signal surface of the disc is projected onto sub-detecting portions D2 and D3 provided at front and rear sides of the main detecting portion D1 of the photo-detector divided into four sensing portions as described previously with reference to FIG. 8, and disclosed in Japanese Patent laid-Open Application Tokkaisho No. 60-209974, and by subjecting to calculation processing, a difference between outputs S5 and S6 from the respective sensors constituting the sub-detecting portions D2 and D3, the tracking deviation of the light spot onto the signal surface for the light beam (main beam) is detected.
More specifically, in the case where the light spots of the light beam are correctly positioned on the signal track of the signal surface, such light spots of the two sub-beams cover the signal track by an equal extent, and thus, the outputs S5 and S6 of the respective sensors for the sub-detecting portions D2 and D3 become equal to each other. When the light spots are positioned in a deviated state at one direction side of the signal track, one of the light spots of the two sub-beams covers a wider (or narrower) range than the other light spot, and thus, the output S5 becomes larger (or smaller) than the output S6, while in the case where the light spots are located in a deviated state at the other direction side of the signal track, one of the light spots of the two sub-beams covers a wider (or narrower) range than the other light spot, and thus, the output S5 becomes smaller (or larger) than the output S6. Therefore, by subjecting to calculation processing, the difference in the outputs of the respective sensors for the sub-detecting portions D2 and D3, the tracking deviation of the light spot for the main light beam can be detected.
Incidentally, it has been a recent trend that CD players are formed into various types, for example, a portable type, a vehicle mounting type, or a type incorporated in a radio-cassette tape recorder, etc. Each of the CD players of the above described types must be compact in size as an appliance itself, since a sufficient space is not available for disposing the optical pickup device as in a stationary type CD player, and thus, the optical pickup device has been reduced in its size, following the diversification in the types of the CD player. Therefore, at the present stage, the optical pickup device based on the astigmatic method in the focusing control through employment of a half-mirror has increasing in number, owing to the fact that the half-mirror can serve the purposes both for a half prism for light path splitting which has been conventionally used, and also for a cylindrical lens having a light condensing capacity only in one direction for producing the astigmatism of the reflected light.
FIG. 9 shows a perspective view of an optical system for a conventional optical pickup device using a half-mirror.
In FIG. 9, the light beam (laser light) generated by a laser diode LD is diffracted by a diffraction grating GD so as to be formed into three beams, i.e. a main beam and two sub-beams, and is thereafter reflected by the surface of a half-mirror M inclined by 45.degree. with respect to an optical axis of the light beam and also, with respect to a record medium, for example, a compact disc CD disposed in parallel relation with respect to said optical axis, and thus, the optical axis is directed to be perpendicular to the signal surface of the disc CD. The light beam thus rejected by the half-mirror M is incident upon an objective lens LO after being formed into a parallel light by a collimator lens LC, and converged by the objective lens LO to be projected onto the signal surface of the disc CD. The light beam thus projected onto the signal surface, is reflected by said signal surface, and returned, as reflected light, to the half-mirror M via the objective lens LO and collimator lens LC so as to be transmitted through said half-mirror M. In the above case, astigmatism is produced in the reflected light depending on the thickness of the half-mirror M, and refraction index and angle of inclination thereof. The reflected light which has transmitted through said half-mirror M is adjusted for the way of generation of astigmatism by a concave lens LV for subsequently being projected onto a photo-detector DP. Here, the photo-detector DP is so disposed that one of the diagonal lines of the main detecting portion D1 thereof (FIG. 8) is aligned with the direction of the optical axis of the light beam directed from the laser diode LD to the half-mirror M. More specifically, the half mirror M is disposed at an angle so that the distance thereof from the disc CD is not constant, and since the reflected light from the disc CD incident upon the half-mirror M has an area, and is of a converged light instead of a parallel light, astigmatism is produced owing to the fact that the angle of incidence of said reflected light is different according to the incident positions thereof upon the half-mirror M, and that the light spot of the reflected light projected onto the photo-detector DP is expanded or contracted in the direction of optical axis and in a vertical direction with respect to said optical axis direction due to the fact that the focusing point of the light beam is deviated form the signal surface of the disc CD.
In the actual construction, the known optical pickup device is generally so arranged that, with an objective lens being disposed within a lens holder (not shown), the lens holder is supported so as to be freely movable in the tracking direction and focusing direction for consequent movement of the objective lens to predetermined positions, and for a support member for supporting the lens holder, said lens holder is formed to be long in a tangential direction of the signal track recorded on the disc. Accordingly, since the lens holder as described above is formed into a rectangular shape or a shape close thereto, having its sides in the above tangential direction and the radial direction of the disc, in the base (not showr) of the optical pickup device for holding various optical parts at predetermined positions, a portion of said base projected to form the optical path from the laser diode LD to the half-mirror M is aligned with the corresponding side portion of said lens holder for the facilitation of designing and also for convenience of forming adjusting openings for adjusting the optical parts.
Incidentally, as disclosed in Japanese Patent Laid-Open Application Tokkaisho No. 57-44236, in the case where the light spot of the light beam is not correctly located on the signal track, i.e. when the tracking is deviated, light intensity distribution within the light spot as projected onto the photo-detector becomes not uniform. This is attributable to the fact that, in the light beam projected onto the signal surface of the disc, light intensity of the reflected light becomes weak at the portion where the light spot of the light beam contacts the pits formed on the signal surface of the disc. Therefore, in the conventional optical pickup device in FIG. 9, during track jumping as in tune searching, the value of the calculation processing for (S1+S3)-(S2+S4) of the outputs S1, S2, S3 and S4 produced from the main detecting portion D1 of the photo-detector shown in FIG. 8 does not become Zero so as to be undesirably regarded as the focusing point deviation of the light beam on the signal surface.
More specifically, since the tangential direction of the signal track of the disc CD at the portion to which the light beam is projected, and the dividing line D1a of the photo-detector form an angle of 45.degree. therebetween, and the light intensity of the light spot of the light beam projected onto the main detecting portion D1 is not uniform, when the light spot is deviated from the center of said main detecting portion D1 due to the track jumping effected, the portion of the light spot strong (or weak) in the light intensity is moved at the angle of 45.degree. with respect to the dividing line D1a of the main detecting portion D1, i.e. in the direction of a line A--A' or B--B' in FIG. 10, and therefore, even in a round circle, the sums of the outputs of the optical sensors on the respective diagonal lines of the main detecting portion D1 become unequal to each other. Accordingly, during the track jumping, even when the focusing is achieved, focusing control is undesirably effected by the difference in the sums of the outputs from the sensors on the respective diagonal lines, consequently giving rise to the focusing deviation of the light beam on the signal surface.